1. Field of the Invention
The present invention relates in general to pumping mechanisms for supplying pressurized fluid to hydraulically actuated systems. In particular, the present invention relates to a gear pump having an inlet port shut-off valve, referred to as a dry valve, having an improved structure for a piston head and rod assembly utilized therein.
2. Description of the Prior Art
Gear pumps are well known in the art and typically include a pair of gears mounted upon respective shafts for rotation within a pump housing. The shafts are arranged such that the gears mesh within a pumping chamber disposed between an inlet port and an outlet port. One of the shafts is rotated by an external source of power so as to cause the two gears to rotate. In this manner, hydraulic fluid is drawn from a reservoir through the inlet port and is discharged at a relatively high pressure from the outlet port to the hydraulic system.
One common use for gear pumps of this type is on a refuse packing vehicle. Such a vehicle is typically driven by an internal combustion engine and includes one or more movable packing mechanisms which are hydraulically actuated. A gear pump can be connected to and driven by the internal combustion engine to generate a flow of pressurized fluid to operate the packing mechanisms. Because of its size and reliability, the gear pump is well suited to perform this function. Typically, however, such packing mechanisms are used only intermittently, requiring no flow of pressurized fluid for long periods of time. The internal combustion engine, on the other hand, is usually continuously operated Thus, for this and other uses, some means must be provided for selectively interrupting the flow of pressurized fluid from the gear pump to the hydraulic system.
One common structure known in the art for accomplishing this selective interruption includes the use of a dry valve. The dry valve is well known in the art and can simply be described as a shut-off valve disposed in the inlet port of the gear pump. When closed, the dry valve obstructs the flow of hydraulic fluid from the inlet port to the pumping chamber of the gear pump. Consequently, the flow of pressurized fluid to the packing mechanisms is interrupted, even though the gear pump is continued to be operated. While the gear pump is operated while the dry valve is closed, it is said to be operating in the dry mode. Typically, means are provided in the dry valve for permitting a relatively small amount of hydraulic fluid to flow into the pumping chamber even when the dry valve is closed. Such relatively small amount of fluid flow is necessary for lubricating and cooling the components of the gear pump while it is operated in the dry mode.
The dry valve typically includes a piston which is movable between opened and closed positions. The piston includes a piston head and a valve head secured to the opposite ends of a piston rod. The valve head is utilized to control the flow of the fluid through the inlet port, while the piston head is utilized to move the piston between the opened and closed positions. Typically, pressurized air is applied to the sides of the piston head to effect such movement. In some instances in the past, the piston head and the piston rod have been formed from a single piece of material. Such an integral structure is wasteful because a large amount of material is discarded during the formation thereof. Also, the concentricity of the head and rod portions of such an integral piston must be maintained precisely in order to insure proper operation. In other instances, the piston head and the piston rod have been formed as separate pieces joined together physically, such as by welding or adhesive, or mechanically, such as by threaded fasteners or cotter pins. Such structures add additional expense and complexity to the piston, both in materials and in assembly labor. Accordingly, it would be desirable to provide an improved piston head and rod structure which avoids all of these problems.